1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive using a rotational position optimization (RPO) algorithm to facilitate write verify operations.
2. Description of the Prior Art
It has been suggested to verify write operations in a disk drive by reading recently written data sectors to verify their recoverability before releasing the cache memory storing the write data. If a recently written data sector cannot be recovered, the write data stored in the cache memory is rewritten to the disk. In effect, the write verify operation detects and compensates for “bad writes” due, for example, to an abnormal fly height or other random anomaly not detected during the write operation. Due to the rotational latency needed to reread a data sector as well as the rotational latency needed to rewrite the data sector, the prior art has suggested various methods for performing the write verify operation so as to minimize the impact on performance.
U.S. Pat. No. 5,872,800 suggests to perform a write verify operation “off-line” while the disk drive is in an idle mode and not processing commands received from the host computer. In this manner, the write verify operation does not impact the performance of the disk drive while processing host commands during normal operation. However, this technique requires a significant amount of cache memory in order to cache the write data for all of the write commands received from the host while the disk drive is “on-line”, as well as the read data associated with read commands received during the same “on-line” period. If the cache memory is exhausted during an “on-line” period, at least some of the write verify commands will be discarded which is undesirable. This problem is exacerbated if the “off-line” periods are short due to frequent access to the disk drive by the host computer.
U.S. Pat. No. 6,289,484 discloses a write verify technique wherein the disk drive performs an “off-line” scan of all previously written data sectors. If during the off-line scan a data sector cannot be recovered on-the-fly using the sector level redundancy, then the data sector is added to a “bad block” list. If during an “on-line” mode the disk drive attempts to write to a data sector in the bad block list, the disk drive will perform an immediate write verify operation on the data sector. In this manner, only the suspect data sectors in the bad block list are verified after a write operation in order to minimize the impact on performance. However, a bad write can occur due to a random anomaly, such as an abnormal fly height, which may not be detected during the off-line scan.
U.S. Pat. No. 5,588,007 discloses a write verify technique wherein the disk drive monitors the on-line write operations for abnormal fly height conditions. If an abnormal fly height condition is detected during a write operation, the disk drive performs an immediate write verify operation on the data sector. However, a bad write may occur due to a random anomaly other than an abnormal fly height.
There is, therefore, the need to improve the write verify operation of a disk drive by minimizing the cache memory requirements while maxi zing the probability that every undetected bad write is corrected.